ie, if we choose a random location and dig down, would the rock strata be in that sequence?
Near-certainly they would be, except in a very few places where something has happened to disturb the order. And we can detect thos cases. Such as the Lewis Overthrust, beloved of creationists, where a huge chunk of landscape was shoved up on top of another chunk of landscape. Creationist ... um ... er ... let's say "fibs" about the Lewis Overthrust are discussed at Claim CD102.1.
and how reliable and consistent are the ages assigned to the different life-forms found in the layers?
Extremely reliable and consistent. The few cases in which there are seeming inconsistencies have been extensively investigated, and almost all those apparent inconsistencies have been found to be consistent after all. Such as the KBS TUff story we've mentioned here.